The present invention generally relates to transmission of a reference laser beam and, in particular, to an apparatus and method for dithering or oscillating a reference laser beam about one or more target areas.
Laser systems have been employed in numerous surveying and construction applications. In one such system, a laser light transmitting device provides a rotating laser beam which establishes a reference plane. Typically, the rotating laser beam is used to define, in effect, a continuous plane of light as a constant horizontal bench mark of elevation over an entire work area. The laser light is either detected by the naked eye or by one or more laser beam detectors placed at considerable distances from the transmitting device.
Various construction tasks can be performed based on the visual perception of the laser light. Where the laser light is to be detected by the naked eye, a reflective target is often positioned in the path of the laser light to improve visibility. Detectors are used to intercept a rotating laser beam and determine an elevation at selected points throughout the work area. Such detectors may provide a display of the beam height for an operator or may form part of a machine control system that controls or assists in the control of various pieces of equipment, such as for example bulldozers and motorgraders.
In a laser light transmitting device, a generally horizontal plane of light is produced by projecting the beam upward and then deflecting the beam ninety degrees within a pentaprism or pentamirror assembly. The pentaprism assembly is rotated about a generally vertical axis within the transmitting device to cause the horizontal beam to sweep around the axis and define the horizontal plane.
U.S. Pat. No. 5,517,023 teaches a transmitter wherein a reference laser beam is dithered at a reduced scanning speed across the face of a reflective target to improve visibility at the target. However, the transmitter taught in the aforementioned patent is limited in several respects. The transmitter disclosed in the '023 patent is capable of dithering across only a single target. It does not provide the capability of scanning multiple targets and moving selectively to previous target positions. Further, the use of reflective targets to establish a dithering reference laser beam in the prior art is problematic because system noise and other interference produced in typical working environments impairs proper detection of target position.
Accordingly, there is a need for a reference laser beam transmitter which enables laser scanning about a plurality of target positions and which provides a means whereby a scanning beam may selectively return to a previous target position, regardless of whether a target lies in the target position. Further, there is a need for a reference laser beam transmitter which reduces the probability of improper target position detection.